Four chemistry classes of acetolactate synthase (ALS)-inhibiting herbicides have been developed and are commercially available for agronomic weed control. They are sulfonylureas such as chlorsulfuron (e.g., GLEAN.RTM. from DuPont), imidazolinones such as imazethapyr (e.g., PURSUIT.RTM. from American Cyanamid Co.), triazolopyrimidines such as flumetsulam (e.g., BROADSTRIKE.RTM. from DowElanco), and pyrimidinylthiobenzoates such as pyrithiobac (e.g., STAPLE.RTM. from DuPont). Because of their low use rates, low mammalian toxicity, good crop selectivity, and high efficacy on target weed species, these herbicides have gained widespread use and popularity in weed control in, e.g., corn, soybeans, wheat, barley, cotton, sorghum, rice, and many other crops. Currently, a wide variety of ALS-inhibiting herbicides are commercially available or under development.
However, weed resistance has recently become a great concern in the industry. Weed resistance to ALS-inhibiting herbicides was first recognized five years after the commercial launch of the sulfonylurea herbicide, chlorsulfuron. See Mallory-Smith et al. Weed Tech. 4:163-168 (1990). Poor rotation of herbicides to dissimilar modes of action, the high efficacy of ALS-inhibiting herbicides, and the relatively high frequency of naturally occurring mutations have been the contributory factors for the widespread weed resistance to ALS-inhibiting herbicides. Many weeds have developed such resistance. In fact, the number of weed species resistant to ALS-inhibiting chemistries is second only to inhibitors of Photosystem II (e.g., triazines). See Heap, Pesticide Science 51:235-234 (1997).
Based on the analysis of the resistant weeds selected from natural weed populations, mutations at five different positions in the primary ALS peptide sequence have been identified to be responsible for ALS-inhibiting herbicide resistance. As summarized in Table 1, each one of the five mutations has a unique cross-class resistance characteristic.
TABLE 1 Cross-Class Resistance to ALS-Inhibiting Herbicides Amino acid ALS-Inhibiting Herbicide Classes position of Imidazo- Triazolo- Pyrimidinylthio- mutation* Sulfonylureas linones pyrimidines benzoates A.sub.122 R P.sub.197 R R A.sub.205 R R R R W.sub.574 R R R R S.sub.653 R R *refers to the amino acid equivalent position in Arabidopsis thaliana
The level of resistance by a weed species having a particular mutation may vary with individual members in one ALS-inhibiting herbicide class. Nevertheless, in general the cross class resistance remains consistent. In other words, if a particular weed is resistant to one member of a particular ALS-inhibiting herbicide class, it will be resistant to all members of that class.
Certain other herbicides which control weeds by a mode of action other than inhibiting acetolactate synthase may be useful in controlling weeds resistant to ALS-inhibiting herbicides. However, none of them possesses all the desirable attributes ALS-inhibiting herbicides have, i.e., low mammalian toxicity, low use rates, good crop selectivity, and high efficacy.